1. Field of the Invention
The present invention relates to an information communication based on Code Division Multiple Access (referred to as "CDMA", hereinafter) scheme and, particularly, to a communication system in which a terminal station performs an intermittent receiving operation during a standby period thereof.
2. Description of the Related Art
A standard IS95 proposed by Qualcomm Inc. has been standardized in the U.S.A., which is known as a typical one of the CDMA scheme. This standardized scheme is publicly opened from TIA (Telecommunication Industries Association) of the U.S.A.
IS95 will be described briefly with reference to FIG. 1. In a base station, a control signal and a traffic signal are similarly spread-spectrum-processed by a spreader 51 and modulated by a modulator 52 and a resultant signal is transmitted from a transmitter (TX) 53 through an antenna 54, as shown in FIG. 1. In a terminal station, the signal received by an antenna 55 and a receiver (RX) 56 is demodulated by a demodulator 57. Output signals of the demodulator 57 are inverse-spread-spectrum-processed by a signal processing circuit 58 to recover the control signal and the traffic signal.
A transmitting part of the base station which employs IS95 will be described in detail with reference to FIG. 2. As shown in FIG. 2, there are a control channel which includes a pilot channel for transmitting a pilot signal, a sync channel for transmitting a sync signal and a paging channel for transmitting a paging signal and a traffic channel for transmitting the traffic signal for controlling a data transfer and a communication signal, as signal channels from the base station to the terminal station. The signals except the pilot signal are input to coders 61 dedicated to the respective channels. Each coder 61 processes the signal input thereto in predetermined manner such as error correcting coding, interleaving, ciphering, etc.
The pilot signal and outputs of the coders 61 are input to respective multipliers 62 to which different spread spectrum codes are also input. The multipliers 62 multiply the pilot signal and the output signals of the corresponding coders 61 with the respective spread spectrum codes to spread frequencies of these signals. The spread spectrum codes used in this stage may be simple short codes and, for example, codes W.sub.x selected from Walsh code system which is well known as the quadrature code system may be used.
The signals whose frequencies are spread by the respective multipliers 62 are input to a combiner 63 and combined with each other thereby. An output of the combiner 63 is branched to an I channel signal and a Q channel signal which are supplied to multipliers 64a and 64b, respectively. The I channel signal is scrambled by multiplying it with a PN code (indicated by "IchPN" in FIG. 2) for the I channel signal by the multiplier 64a. The Q channel signal is scrambled by multiplying it with a PN code (long code and indicated by "QchPN" in FIG. 2) for the Q channel signal by the multiplier 64b.
The scrambled I and Q signals whose high harmonics are removed by respective low-pass filters 65 are input to a quadrature modulator 66. The quadrature modulator 66 phase-modulates a carrier by using the I and Q signals input thereto. A signal which is phase-modulated is transmitted from a transmitter 67 through an antenna 68.
Now, a receiving part of the terminal station employing IS95 will be described in detail with reference to FIG. 3.
In the terminal station, a receiving signal received by an antenna 71 and a receiver (RX) 72 is input to a quadrature demodulator 73, as shown in FIG. 3. The quadrature demodulator 73 quadrature-demodulates the receiving signal to recover the I channel signal and the Q channel signal of a base-band signal. The demodulated I and Q channel signals are input to respective low-pass filters 74. The I and Q channel signal output from the low-pass filters 74 are input to respective A/D converters 75 by which the signals are converted into digital signals.
A digital signal processing circuit 76 decodes the control signal and the traffic signal from the input digital signals under control of a CPU 77. In detail, the digital signal processing circuit 76 finds out a timing of the pilot signal by means of a pilot search engine. With the timing thus found, it is possible to establish a synchronization of timings of the scrambles by the PN code for the I channel signal and the PN code for the Q channel signal.
Thereafter, the sync signal is decoded by performing an inverse spreading and Rake synthesis on the basis of the timing thus found. With this, it becomes possible to establish a frame synchronization and the spreading code used in the paging channel is clarified.
Then, the signal of the paging channel is decoded by using the spreading code thus clarified. In the standby state, the CPU 77 continuously monitors the paging channel. In the communication state, the signal on the traffic channel is decoded on the basis if the information on the paging channel.
In IS95, however, it is difficult to reduce a power consumption since the digital signal processing circuit has to be always operable to trace the pilot signal and to monitor the paging channel, even when the terminal station is in the standby state. Further, since the pilot signal is continuously traced and the synchronization is to be held, a complete intermittent operation is impossible in the standby state.
Other prior arts of the CDMA communication system than IS95 are disclosed in Japanese Patent Application Laid-open Nos. Hei 1(1989)-305741, Hei 5(1993)-130069 and Hei 5(1993)-191375.
Japanese Patent Application Laid-open No. Hei 1(1989)-305741 discloses a spectrum spreader in which an information to be transmitted and a sync signal are frequency-spread by different spread codes and the diffused information and the sync signal are multiplexed and transmitted. The disclosed technique is also utilized by IS95 and can not solve the above mentioned problem. Japanese Patent Application Laid-open No. Hei 5(1993)-130069 discloses a multiple connection control device for reducing power consumption. This device is constructed such that sync catching/holding means are provided correspondingly to a plurality of control signals, respectively, to continuously monitor all of the control signals and, thus, it can not realize an intermittent operation. Although Japanese Patent Application Laid-open No. Hei 5(1993)-191375 also discloses a receiver of a spectrum spread system for the purpose of reducing power consumption during an intermittent operation, there is neither description nor suggestion of reduction of power consumption during intermittent operation in its specification.
The present invention has an object, in the CDMA communication system, to realize a complete intermittent operation of a terminal station during a standby state while reducing power consumption during the intermittent operation.